1. Field of the Invention
There are an increasingly large number of genes available for expression, where the expression product may find commercial use. In many instances, the initial expression has been observed in E. coli. Expression in E. coli has many disadvantages, one in particular being the presence of an enterotoxin which may contaminate the product and make it unfit for administration to mammals. Furthermore, there has not previously been an extensive technology concerned with the production of products in E. coli, as compared to such other microorganisms as Bacillus subtills, Streptomyces, or yeast, such as Saccharomyces.
In many situations, for reasons which have not been resolved, heterologous products, despite active promoters and high copy number plasmids, are produced in only minor amount, if at all, in a microorganism host. Since the economics of the processes are dependent upon a substantial proportion of the nutrients being employed in the expression of the desired product, the production of these products in unicellular microorganisms appears to be unpromising. There is, therefore, a substantial need for processes and systems which greatly enhance the production of a desired polypeptide without substantial detriment to the viability and growth characteristics of the host.
2. Description of the Prior Art
Villa-Komaroff et al., Proc. Natl. Acad. Sci. USA (1978) 75:3727-3731, describe a fusion sequence encoding proinsulin joined to the N-terminus of penicillinase for expression in E. coli. Paul et al., European J. Cell Biol. (1983) 31:171-174, describe a fusion sequence encoding proinsulin joined to the COOHterminus of a portion of the tryptophan E gene product for expression in E. coli. Goeddel et al., ibid. (1979) 76:106-110, describe synthetic genes for human insulin A and B chains fused to E. coli .beta.-galactosidase gene to provide a fused polypeptide in E. coli. Stepien et al., Gene (1983) 24:289-297, describe expression of insulin as a fused product in yeast, where the proinsulin gene was fused to the N-terminus coding sequence of GALl for expression in yeast.